In forming a network using the optical signal modulated by the data signal, it is necessary to monitor the quality of the optical signal propagating through the network with the optical signal monitoring apparatus.
Generally, the optical signal monitoring apparatus obtains information on a waveform of a signal on a transmission path, and computes a value indicating the signal quality from the waveform information and supplies the computation result. Therefore, in the optical signal monitoring apparatus, it is extremely difficult to obtain the waveform information by a photodetector directly receiving the optical signal which is modulated by a data signal having a bit rate very high as tens of Gb/s.
In a conventional optical signal monitoring apparatus, an equivalent time sampling method is adopted to obtain the information on the waveform of the high-speed optical signal.
In the equivalent time sampling method, sampling is performed to an optical signal P in which the same waveform is repeated at a period Ta as shown in (a) of FIG. 9 using a sampling pulse S having a period of Ts=N·Ta+ΔT which is slightly longer than N multiplications (N is an integer) of the period Ta by a time ΔT as shown in (b) of FIG. 9, thereby obtaining instantaneous amplitude values (instantaneous intensity) at positions shifted from each other by ΔT in the repetitive waveform of the optical signal P as shown (c) of in FIG. 9.
A waveform P′ drawn by an envelope curve connecting the obtained instantaneous amplitude values is one in which the waveform of the optical signal P is enlarged by a factor of Ts/ΔT on a temporal axis. The waveform P′ retains the waveform characteristics of the original optical signal P.
Accordingly, a probability distribution of an amplitude indicating one of binary levels and an amplitude indicating another of the binary levels is obtained for the waveform information obtained by the equivalent time sampling, and a Q value indicating the signal quality can be obtained by computation of a standard deviation of the probability distribution.
For example, Patent Document 1 discloses a technique of performing the equivalent time sampling to the optical signal to compute the Q value indicating the signal quality from the obtained waveform information.    Patent Document 1: Japanese Patent No. 3796357
In the technique disclosed in Patent Document 1, usually a non-linear optical material is used as an element for performing the equivalent time sampling to the optical signal.
Because the non-linear optical material usually has low sampling efficiency (sampling efficiency measured by a waveform conversion phenomenon, that is, waveform conversion efficiency is not more than −20 dB), high S/N can be obtained for the information on the waveform of the strong optical signal while poor S/N is obtained for the information on the waveform of the weak optical signal.
Therefore, it is thought that an electroabsorption optical modulator having a transmission loss which is significantly lower than that of the non-linear optical material, in other words, the substantially high sampling efficiency is used as the element for performing the equivalent time sampling of the optical signal.
The electroabsorption optical modulator has a characteristic in which an absorption index is changed with respect to light passed through an optical path according to a level of an electric field imparted to the optical path connecting two optical terminals. In the electroabsorption optical modulator, the optical signal is fed into one of the optical terminals, an electric sampling pulse signal is imparted to a power supply terminal, and the absorption index is lowered with respect to the optical signal to supply the sampling pulse signal from another of the optical terminals only when the sampling pulse signal is fed, thereby performing the sampling to the optical signal.
For example, Patent Document 2 discloses a technique of imparting the electric sampling pulse signal to the power supply terminal of the electroabsorption optical modulator to perform the sampling to the optical signal.    Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2004-222252
However, in the technique disclosed in Patent Document 2, high-frequency impedance matching is required in the electric sampling pulse signal, and it is extremely difficult that a pulse width of the electric sampling pulse signal is stably narrowed without ringing. Therefore, there is generated a new problem in that the information on the waveform of the optical signal modulated at tens of Gb/s cannot accurately be obtained with sufficient resolution.
Furthermore, in the optical signal quality monitoring apparatus in which the equivalent time sampling method disclosed in Patent Document 1 is adopted, the obtained waveforms of the predetermined number of bits are superimposed to produce an eye pattern. In the case where the quality computation processing is performed to the eye pattern, a temporal axes of the superimposed waveforms are gradually shifted unless the sampling period Ts is strictly matched with N·Tc+ΔT, which results in another problem in that the quality computation cannot correctly be performed.